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| United States Patent |
5,534,209
|
|
Moriya
|
July 9, 1996
|
Method for manufacturing a liquid crystal polymer film and a liquid
crystal polymer film made thereby
Abstract
The invention is for a process to manufacture liquid crystal polymer films
having balanced physical properties. The process includes feeding an
oriented liquid crystal polymer material into a melt region formed between
support membranes passing over the surface of and through an opening
between heated rolls. The orientation of the liquid crystal polymer
material is randomized and passed through the opening with the support
membranes to form a laminated sandwich structure consisting of an
unoriented liquid crystal polymer film between the support membranes. In
another embodiment of the invention laminated sandwich structure is
stretched to impart multiaxial orientation to the liquid crystal polymer
film. The support membranes can be optionally removed to produce a film
having a desirable surface finish.
| Inventors:
|
Moriya; Akira (Okayama, JP)
|
| Assignee:
|
Japan Gore-Tex, Inc. (JP)
|
| Appl. No.:
|
403516 |
| Filed:
|
March 13, 1995 |
Foreign Application Priority Data
| Current U.S. Class: |
264/171.13; 264/175; 428/1.6 |
| Intern'l Class: |
B29C 043/24 |
| Field of Search: |
264/171.13,172.19,173.1,175
428/1
|
References Cited
U.S. Patent Documents
| 3953566 | Apr., 1976 | Gore.
| |
| 3962153 | Jun., 1976 | Gore.
| |
| 4096227 | Jun., 1978 | Gore.
| |
| 4187390 | Feb., 1980 | Gore.
| |
| 4802061 | Jan., 1989 | Portugall et al. | 361/400.
|
| 4923660 | May., 1990 | Willenberg et al. | 264/103.
|
| 4956140 | Sep., 1990 | Rolles et al. | 264/175.
|
| 5039208 | Aug., 1991 | Ohnishi et al.
| |
| 5186960 | Feb., 1993 | Walsh, Jr.
| |
| 5238523 | Aug., 1993 | Yuasa et al.
| |
| 5384168 | Jan., 1995 | Dubal et al.
| |
| Foreign Patent Documents |
| 137449 | Apr., 1985 | EP | 264/175.
|
| 0484818 | May., 1992 | EP.
| |
| 0512397 | Nov., 1992 | EP.
| |
| 0612610 | Aug., 1994 | EP.
| |
| 60-172306 | Sep., 1985 | JP.
| |
| 63-031729 | Feb., 1988 | JP.
| |
| 1-130390 | May., 1989 | JP.
| |
| 2-089616 | Mar., 1990 | JP.
| |
| 2-089617 | Mar., 1990 | JP.
| |
| 2-175737 | Jul., 1990 | JP.
| |
| 2-178016 | Jul., 1990 | JP.
| |
| 02-203321 | Oct., 1990 | JP.
| |
| 3-152131 | Jun., 1991 | JP.
| |
| 4-62144 | Feb., 1992 | JP | 264/175.
|
| 4-166309 | Jun., 1992 | JP.
| |
| 4-308737 | Oct., 1992 | JP.
| |
| 5-043664 | Feb., 1993 | JP.
| |
| 2166685 | May., 1986 | GB.
| |
Primary Examiner: Timm; Catherine
Attorney, Agent or Firm: Samuels; Gary A.
Claims
I claim:
1. A method for manufacturing a liquid crystal polymer film comprising the
steps of:
(a) feeding a thermotropic liquid crystal polymer material into a melt
region formed between opposed inward-facing surfaces of two support
membranes,
each support membrane, in the melt region, having an outward-facing surface
in contact with the surface of a heated roll,
said heated rolls spaced apart to form an opening between the rolls and a
gap between said support membranes passing over said rolls;
(b) heating and maintaining said liquid crystal polymer material at a
temperature at or above its melt point and accumulating sufficient liquid
crystal polymer material in the melt region above said gap to form a
reservoir of melted liquid crystal polymer material wherein said liquid
crystal polymer becomes randomly oriented;
(c) passing said support membranes and liquid crystal polymer material
through said opening between said rolls to form a sandwich structure
comprising a liquid crystal polymer film between said support membranes,
the width of said opening adjusted to preserve the random orientation of
said liquid crystal polymer material forming said liquid crystal polymer
film;
(d) solidifying said liquid crystal polymer film.
2. The method for manufacturing a liquid crystal polymer film as recited in
claim 1 further comprising the step of removing the support membrane from
one or both surfaces of the liquid crystal polymer film.
3. The method for manufacturing a liquid crystal polymer film as recited in
claim 1 further comprising the step of providing as at least one of the
support membranes, a nonporous membrane.
4. The method for manufacturing a liquid crystal polymer film as recited in
claim 1 further comprising the step of providing as at least one of the
support membranes, a porous membrane.
5. The method for manufacturing a liquid crystal polymer film as recited in
claim 4 further comprising the step of providing as at least one of the
support membranes, a porous polytetrafluoroethylene membrane.
6. A method for manufacturing a liquid crystal polymer film comprising the
steps of:
(a) feeding a thermotropic liquid crystal polymer material into a melt
region formed between opposed inward-facing surfaces of two support
membranes,
each support membrane, in the melt region, having an outward-facing surface
in contact with the surface of a heated roll,
said heated rolls spaced apart to form an opening between the rolls and a
gap between said support membranes passing over said rolls;
(b) heating and maintaining said liquid crystal polymer material at a
temperature at or above its melt point and accumulating sufficient liquid
crystal polymer material in the melt region above said gap to form a
reservoir of melted liquid crystal polymer material wherein said liquid
crystal polymer becomes randomly oriented;
(c) passing said support membranes and liquid crystal polymer material
through said opening between said rolls to form a sandwich structure
comprising a liquid crystal polymer film between said support membranes,
the width of said opening adjusted to preserve the random orientation of
said liquid crystal polymer material forming said liquid crystal polymer
film;
(d) stretching said sandwich structure in at least one direction, at a
temperature at or above the melt point of the liquid crystal polymer,
thereby imparting multiaxial orientation to the liquid crystal polymer
film; and
(e) solidifying said liquid crystal polymer film.
7. The method for manufacturing a liquid crystal polymer film as recited in
claim 6 further comprising the step of stretching said sandwich structure
in at least two directions, at a temperature at or above the melt point of
the liquid crystal polymer, thereby imparting multiaxial orientation to
the liquid crystal polymer film.
8. The method for manufacturing a liquid crystal polymer film as recited in
claim 6 further comprising the step of solidifying the liquid crystal
polymer film before stretching the sandwich structure.
9. The method for manufacturing a liquid crystal polymer film as recited in
claim 6 further comprising the step of removing the support membrane from
one or both surfaces of the liquid crystal polymer film.
10. The method for manufacturing a liquid crystal polymer film as recited
in claim 6 further comprising the step of providing as at least one of the
support membranes, a nonporous membrane.
11. The method for manufacturing a liquid crystal polymer film as recited
in claim 6 further comprising the step of providing as at least one of the
support membranes, a porous membrane.
12. The method for manufacturing a liquid crystal polymer film as recited
in claim 11 further comprising the step of providing as at least one of
the support membranes, a porous polytetrafluoroethylene membrane.
Description
FIELD OF THE INVENTION
The invention relates to a method for making films of high molecular weight
liquid crystal polymers and to liquid crystal polymer (LCP) films made
according to the method.
BACKGROUND OF THE INVENTION
Liquid crystal polymers are a family of materials that exhibit a highly
ordered structure in the melt, solution, and solid states. They can be
broadly classified into two types; lyotropic, having liquid crystal
properties in the solution state, and thermotropic, having liquid crystal
properties in the melted state.
Most liquid crystal polymers exhibit excellent physical properties such as
high strength, good heat resistance, low coefficient of thermal expansion,
good electrical insulation characteristics, low moisture absorption, and
are good barriers to gas flow. Such properties make them useful in a broad
range of applications in the form of fibers, injection molded articles,
and, in sheet form, as electronic materials for printed circuit boards,
packaging, and the like.
Many of the physical properties of liquid crystal polymers are very
sensitive to the direction of orientation of the liquid crystal regions in
the polymer. The ordered structure of the liquid crystal polymer is easily
oriented by shear forces occurring during processing and highly aligned
liquid crystal chains can be developed that are retained in the solid
state, and result in highly anisotropic properties. This can be highly
desirable for certain products, for example, in filaments, fibers, yarns,
and the like. Anisotropic properties are often not desirable, however, in
products having planar forms, such as tape, films, sheet, and the like.
A number of methods are used to produce liquid crystal polymer materials in
planar forms that have more balanced, less anisotropic properties. These
include the use of multilayer flat extrusion dies which are fashioned such
that they extrude overlapping layers at intersecting angles, use of static
mixer-agitators at the die inlets, and the like. More recently, dies
having rotating or counter-rotating surfaces have become known in the art
and successfully used. These extrusion techniques, used separately or in
combination with other methods known in the art, such as film blowing, can
produce liquid crystal polymer film and sheet that are multiaxially
oriented, that is, oriented in more than one direction, and have more
balanced physical properties.
A characteristic of these methods is that locally, for example, at the
surfaces of the sheet or film, the molecules are oriented in the planar
x-y directions by shear imparted at the extrusion surfaces. In the
z-direction, i.e., throughout the thickness, the x-y orientation of the
molecules will change progressively from the orientation at one surface to
the orientation at the other surface of the planar form. A drawback to
these methods is that when attempting to make very thin multiaxially
oriented films, e.g., films having a thickness of 25 micrometers or less,
the forces imparted in the orientation transition region of the liquid
crystal polymer by the extrusion surfaces are exerted in increasingly
opposing directions as the distance between the extrusion surfaces
diminishes and the formation of pinholes, tears, and other imperfections
in the film takes place.
Additionally, and particulary in the case of melt-processed thermotropic
liquid crystal polymers which have very high processing viscosity, it is
difficult to obtain films with uniform surface smoothness and thickness by
the processes described above. Problems such as film curling, streaking,
and tear sensitivity have been associated with surface roughness and
irregularities or nonuniform thickness in the films.
SUMMARY OF THE INVENTION
This invention provides a method for making a liquid crystal polymer film
in which the liquid crystal polymers are randomly or, alternatively,
multiaxially oriented; and which can produce a film having remarkably
uniform surface finish and thickness. Furthermore, the films thus produced
are strong, have balanced physical properties in at least the machine and
transverse directions, and are free of tendencies to curl or surface peel.
The method comprises the steps of:
(a) feeding a thermotropic liquid crystal polymer material into a melt
region formed between opposed inward-facing surfaces of two support
membranes,
each support membrane, in the melt region, having an outward-facing surface
in contact with the surface of a heated roll,
the heated rolls spaced apart to form an opening between the rolls and a
gap between the support membranes passing over the rolls;
(b) heating and maintaining the liquid crystal polymer material at a
temperature at or above its melt point and accumulating sufficient liquid
crystal polymer material in the melt region above the gap to form a
reservoir of melted liquid crystal polymer material wherein the liquid
crystal polymer material becomes randomly oriented;
(c) passing the support membranes and liquid crystal polymer material
through the opening between the rolls to form a sandwich structure
comprising a liquid crystal polymer film between the support membranes,
the width of the opening adjusted to preserve the random orientation of the
liquid crystal polymer material forming the liquid crystal polymer film;
(d) solidifying the liquid crystal polymer film.
Another embodiment of the invention comprises the method described above
and further comprises the step of stretching the sandwich structure in at
least one direction, at a temperature at or above the melt point of the
liquid crystal polymer, thereby imparting orientation to the liquid
crystal polymer film.
Yet another embodiment of the invention comprises either of the methods
above and further comprises the step of removing the support membrane from
one or both surfaces of the liquid crystal polymer film.
It is recognized that "membrane" and "film" can often be used
interchangeably, however, to avoid confusion, "membrane" will generally be
used herein with respect to the porous support material; and "film" with
respect to liquid crystal polymer material.
Liquid crystal polymer, as used herein, is meant to include polymer alloys
having a liquid crystal polymer component as well as liquid crystal
polymers alone. For convenience, the term "liquid crystal polymer" is used
herein to include material of both kinds.
By multiaxially oriented liquid crystal polymer, as used herein, is meant
liquid crystal polymer material to which forces in more than one direction
have been applied in order to orient the liquid crystal polymer.
By porous as used herein is meant a structure of interconnected pores or
voids such that continuous passages and pathways throughout a material are
provided.
Longitudinal direction, x-direction, and machine direction (MD) as used
herein indicates the planar direction of manufacture of a film or sheet;
transverse direction (TD) and y-direction indicate the planar direction
normal to the direction of manufacture.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of an embodiment of the invention.
FIG. 2 is a schematic drawing of another embodiment of the invention.
FIG. 3 is a photomicrograph (400.times.magnification) of a portion of the
surface of a liquid crystal polymer film made according to Example 5.
FIG. 4 is a photomicrograph (4000.times.magnification) of a portion of the
surface of a liquid crystal polymer film shown in FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
With reference to the drawings, the process of the invention will be
described in detail.
In FIG. 1 is shown a thermotropic liquid crystal polymer material 1 being
fed from an extruder 2 into a melt region 4,4' formed between support
membranes 3,3' where the support membranes pass over the surface of heated
rolls 6,6'. The liquid crystal polymer 1 is fed at a rate sufficient to
create and maintain the level of a reservoir 8, or puddle, of melted
liquid crystal polymer in the melt region 4,4' above the nip of the heated
rolls 6,6'. By "nip" is meant the location of minimum distance between the
roll surfaces. The width of the opening at the nip is adjusted so that
support membranes 3,3' and a portion of the liquid crystal polymer 1 in
the reservoir 8 are brought together and adhered to form a sandwich
structure 5 consisting of a liquid crystal polymer film between the
support membranes. The sandwich structure 5 can then, optionally, be
cooled to solidify the liquid crystal polymer film by conventional means
(not shown) such as passage over a chill roll, air-cooled, or the like,
and taken up.
Another embodiment of the process is a step in which one or both support
membranes is removed from the surface of the sandwich structure 5. One or
both of the support membranes can be removed or peeled from the sandwich
structure by conventional web-handling means, including removal by hand.
Therefore, the process step depicted in FIG. 2 is exemplary only and is
intended to be not limiting.
In FIG. 2 is shown a step in which both support membranes are removed from
the sandwich structure. As shown in FIG. 2, the sandwich structure 5
comprising a solidified liquid crystal polymer film is passed between peel
rolls 7,7'. The support membranes 3,3' are peeled from the surfaces of the
sandwich structure 5 as they exit the nip of the peel rolls and are
collected on support membrane take-up rolls 9,9'. The now unsupported
free-standing liquid crystal polymer film 15 is collected on main take-up
roll 10.
Materials for the support membrane of the invention are selected on the
basis of their ability to withstand the forces and temperatures of
processing, on their ability to be stretched, and on their chemical
resistance to the liquid crystal polymers and solutions with which they
are combined. The support membrane is preferably made of a synthetic
polymer and may be porous or nonporous.
Suitable synthetic polymers for nonporous support membranes include
thermoplastic polyimides, polyethersulfones, polyethylene terephthalates,
and the like. Such membranes are well known in the art, and are
commercially available from many sources. The nonporous support membranes
should be in the range 10 to 500 micrometers thick, preferably 50 to 200
micrometers thick.
Porous sheets or membranes of synthetic polymers, for example,
polyethylene, polypropylene, or other polyolefins; polyesters,
polycarbonates, polystyrenes, polyvinyl chloride, or fluoropolymers, and
the like, can also be used as support membranes. Porous support membranes
should have an average pore size in the range 0.05 to 5.0 micrometers,
preferably 0.2 to 1.0 micrometers; a pore volume in the range 40 to 95
percent, preferably 60 to 85 percent; and a thickness in the range 5 to
300 micrometers, preferably 20 to 150 micrometers. Fluoropolymers,
including tetrafluoroethylene/(perfluoroalkyl) vinyl ether copolymer
(PFA), tetrafluoroethylene/hexafluoropropylene copolymer (FEP), and
polytetrafluoroethylene (PTFE), and the like,are preferred for their
processing characteristics, temperature resistance, and chemical
inertness. Most preferred are porous membranes of polytetrafluoroethylene.
Suitable porous polytetrafluoroethylene membranes can be made by processes
known in the art, for example, by papermaking processes, or by processes
in which filler materials are incorporated with the PTFE resin and are
subsequently removed to leave a porous structure. Preferably the porous
polytetrafluoroethylene membrane is porous expanded
polytetrafluoroethylene membrane having a structure of interconnected
nodes and fibrils, as described in U.S. Pat. Nos. 3,953,566, 3,962,153,
4,096,227, and 4,187,390 which fully describe the preferred material and
processes for making them.
Thermotropic liquid crystal polymers, because of their melt processibility,
are preferred as the liquid crystal polymer of the invention. Virtually
any kind of thermotropic liquid crystal polymer material can be made into
a film by the method of the invention, and selection of a suitable liquid
crystal polymer material is based on the end use projected for the film.
The method is particularly well suited for processing high molecular
weight high melt point thermotropic liquid crystal polymers, especially
those having a melt point of 200.degree. C. or higher. Suitable
thermotropic liquid crystal polymers include aromatic polyesters which
exhibit liquid crystal properties when melted and which are synthesized
from aromatic diols, aromatic carboxylic acids, hydroxycarboxylic acids,
and other like monomers. Typical examples include a first type consisting
of parahydroxybenzoic acid (PHB), terephthalic acid, and biphenol; a
second type consisting of PHB and 2,6-hydroxynaphthoic acid; and a third
type consisting of PHB, terephthalic acid, and ethylene glycol. They are
represented below as Formulas 1, 2, and 3 respectively.
##STR1##
Also, in the present invention, a polymer alloy having a liquid crystal
polymer component can be used. In such cases the polymer which is mixed
with or chemically bonded to a liquid crystal polymer can be selected from
the group consisting of, but not limited to, polyetheretherketones,
polyether sulfones, polyimides, polyamides, polyacrylates, and the like.
The polymers and liquid crystal polymer components are mixed in a weight
ratio of 10:90 to 90:10, preferably in the range of 30:70 to 30:70.
The liquid crystal polymers and polymer alloys described hereinabove are
meant for illustration and not for limitation of the invention. It is
recognized by the inventor that many other liquid crystal polymers and
polymer alloys suitable for use in the invention are known in the art.
Likewise, it is recognized that compatibilizers, plasticizers, flame
retardant agents, and other additives may be included with the liquid
crystal polymers.
Referring now to FIG. 1, a melt region 4,4' is formed between the inward
facing surfaces of support membranes 3,3' where the support membranes pass
over and contact the curved surfaces of the upper facing quadrants of
heated calender rolls 6,6'. The rolls are disposed so that their axes are
parallel and horizontal and, therefore, the distance between the rotating
curved surfaces of the rolls, measured progressively from the top to the
bottom of the upper facing quadrants, continuously diminishes until the
nip, or minimum distance, between the surfaces at the bottom of the upper
facing quadrants is reached. The curved surfaces are thus disposed to
provide in the melt region an upward-facing chamber, roughly V-shaped in
cross-section, having a lower minimum opening dimension defined by the
width of the opening at the nip and an upper maximum opening dimension
much larger than the minimum dimension. Other types of heating surfaces
can also be used, for example, angled flat platens or platens having
curved surfaces, so long as they are arranged to provide the features
described above, however, rolls are preferred for their availability and
ease of use.
Again referring to FIG. 1, a thermotropic liquid crystal polymer material 1
is fed from an extruder 2, for example, through a conventional T-die, into
the melt region 4,4'. The liquid crystal polymer material 1 arrives in
melted form and is oriented in the machine direction by shear imparted by
the die. The liquid crystal polymer material is initially fed at a rate
such that a puddle of melted liquid crystal polymer material to form a
reservoir 8 is accumulated above the gap between the support membranes
3,3' passing over the surfaces and through the nip of heated rolls 6,6'.
The reservoir 8 formed above the nip of the heated rolls has a capacity
sufficient to contain a much greater volume of melted liquid crystal
polymer than instantaneously passes through the nip. When a sufficient
amount of melted liquid crystal polymer material has been accumulated in
the reservoir, the process rates are adjusted by decreasing the material
feed rate or increasing roll speed so that the amount of melted liquid
crystal polymer material contained in the reservoir is kept essentially
constant. An alternative feed method is to introduce the liquid crystal
polymer material 1 into the melt region 4,4' as a solid oriented film,
fiber, rod, or the like. In this case, the liquid crystal polymer material
is initially melted in the melt region, although it may be preheated to
facilitate melting.
The heated rolls are heated to a temperature sufficient to maintain the
thermotropic liquid crystal polymer material in the melt region at least
at a temperature at or above the melt point of the liquid crystal polymer,
typically the temperature of the liquid crystal polymer material in the
melt region will be in the range 10.degree. to 50.degree. C. or more above
the melt point of the liquid crystal polymer. Preferably the liquid
crystal polymer, which enters the top of the reservoir in the melt region
in an oriented state, is heated to a temperature which lowers the
viscosity of the melted material and permits it to become randomly
oriented. The purpose for the reservoir of melted liquid crystal polymer
material is to provide a place and sufficient residence time at
temperature for the material to relax from its oriented feed state, in
which the liquid crystal polymers are aligned substantially in one
direction, to a random unoriented state before the material and support
membranes are passed through the nip of the heated rolls to form a
laminated sandwich structure.
The randomly oriented liquid crystal polymer material and support membranes
3,3' are passed from the melt region 4,4' through the opening at the nip
of heated rolls 6,6' to form the laminated structure 5. The width of the
opening at the nip and the compressive forces applied to the support
membranes are adjusted to minimize disturbance of the random orientation
of the liquid crystal polymer material. Only light compressive force is
used in order to prevent movement of the liquid crystal polymer material
independent of the support membrane. The bond strength between the liquid
crystal polymer material and the support membranes is influenced by the
adhesive affinity the materials have for each other, and some
experimentation may be required to select appropriate material
combinations. In the case of porous support membranes bonding is
facilitated by mechanical interlocking of the liquid crystal polymer
material with the pore structure of the support membranes. This mechanical
interlocking is particularly important when using porous fluoropolymer
support membranes as fluoropolymers are well known for their release
properties. However, by selection of the materials and properties of
porous support membranes, for example pore size and pore volume; and by
controlling process variables such as the temperature and viscosity of the
material in the melt region, process speed, and the like, which influence
the depth of penetration of the liquid crystal polymer material into the
porous support membrane, bonding can be readily achieved and bond strength
easily controlled.
The support membranes 3,3' move through the nip at the same rate as the
liquid crystal polymer material and impart no shear forces to the liquid
crystal polymer material adhering to and forming a film between them. The
gap between the support membranes 3,3' is sufficiently wide that, as much
as possible, the velocity of the liquid crystal polymer material exiting
the melt region is kept uniform and virtually no shear is imparted to the
liquid crystal polymer material, thus preserving the random orientation of
the liquid crystal polymer film of the laminated sandwich structure 5. The
laminated sandwich structure 5 can then be cooled to solidify the liquid
crystal polymer film by conventional means, such as passage over chill
rolls, forced air cooling, or the like, and taken up. The liquid crystal
polymer film thus produced is an essentially unoriented film having
balanced physical properties, at least in the planar x-y directions, and
having remarkably uniform thickness.
In another embodiment of the invention a step is added to the process
described above in which one or both support membranes 3,3' can be removed
from the surface of the laminated sandwich structure 5. An example of this
step is shown in FIG. 2 which depicts peeling and removing both support
membranes from the laminated sandwich structure to produce a free-standing
liquid crystal polymer film having the properties described above. It has
been discovered that peeling the support membranes from the liquid crystal
polymer film can produce a microscopic texture on the film surface. Again,
by selection of the materials and properties of the support membranes, for
example pore size and pore volume of porous support membranes; and by
controlling process variables which also influence the support membrane-to
liquid-crystal-polymer bonding characteristics such as melt temperature,
viscosity, process speed, and the like, the microscopic texturizing of the
film surface can be controlled to produce a uniform surface finish in the
range 0.05 to 5 micrometers, i.e., from very smooth to a microscopic
roughness, which is very useful for improving surface related properties
in the liquid crystal polymer film, for example, increased surface area
for bonding by adhesives, metal-plating properties, and the like. An
example of such microscopic texturizing obtained using a porous expanded
polytetrafluoroethylene membrane having a nominal pore size of 1.0
micrometers and a pore volume of about 80% is shown in the
photomicrographs of FIGS. 3 and 4.
When no additional melting steps are required to produce a liquid crystal
polymer film for an end-use application, it is desirable to heat treat the
film to stabilize dimensional and physical properties, and reduce strains
in the liquid crystal polymer film. The heat treatment can be done at any
time after the liquid crystal polymer film has been solidified, and with
or without the support membranes in place. The heat treatment is done with
the material restrained to prevent shrinking. Heat treatment temperature
will vary according to the thermotropic liquid crystal polymer used, but
is generally in the range from slightly above the glass transition
temperature (Tg) to slightly below the melt range of the liquid crystal
polymer.
In yet another embodiment of the invention a stretching step is included in
the process, in which the laminated sandwich structure 5 produced by the
process described hereinabove is stretched in at least one direction,
preferably in at least two directions, to impart multiaxial orientation to
the unoriented liquid crystal polymer film. An advantage obtained by using
the laminated sandwich structure comprising an unoriented liquid crystal
polymer film, produced as described above, as the starting material for
the stretching step, is that orientation of the liquid crystal polymer
material ordinarily developed in steps prior to stretching, such as
extrusion, blowing, or calendering steps, need not be accommodated or
compensated for, as is the case with conventional processes used to
develop balanced properties in stretched liquid crystal polymer films.
In the stretching step the laminated sandwich structure 5 formed of the
thermotropic liquid crystal polymer film and porous support membrane is
heated to a temperature at or above the melt point of the liquid crystal
polymer and, preferably stretched in at least two directions. Stretching
in at least two directions may be done simultaneously or sequentially, and
may be done in one or more steps. The amount of stretch, relative to
original planar x-y directions or radial directions, is ordinarily in the
range 1.5 to 15:1, preferably in the range 4 to 8:1. The rate of stretch
is generally in the range 5% to 500% per minute, preferably in the range
20% to 100% per minute; at a speed in the range 0.2 to 10 meters/minute,
preferably in the range 1 to 6 meters/minute.
Stretching may be done using conventional equipment or apparatus known in
the art. For example, multiaxial simultaneous stretching can be done using
a radial stretching pantograph; and biaxial stretching in the planar x-y
directions can be done, simultaneously or sequentially, using an x-y
direction stretching pantograph. Also, equipment having sequential
uniaxial stretching sections can be used, for example, a machine having a
section containing differential speed rolls for stretching in the machine
direction (MD), and a tenter frame section for stretching in the
transverse direction (TD).
As the laminated sandwich structure is stretched in the planar x-y
directions the area of its surface progressively increases and its
thickness is progressively reduced. The melted liquid crystal polymer
film, bonded to and supported by the porous support membrane, is also
stretched by the stretching membrane, also increases in area in the planar
x-y directions, and the liquid crystal polymers of the film become
oriented by the stretching. At the same time, as there is no change in the
volume of liquid crystal polymer, the liquid crystal polymer film becomes
progressively thinner as the material of the film is dragged and spread by
the stretching membrane.
By controlling the amount of stretch, rate of stretch and directions of
stretch of the laminated sandwich structure, the liquid crystal polymers
can be aligned in a preferred orientation, or aligned multiaxially and
more randomly to provide high strength films having relatively balanced
physical properties in the planar x-y directions. In general, liquid
crystal polymer films having balanced physical properties are more useful,
particularly for electronic applications such as printed circuit boards,
and are preferred. By controlling the amount of liquid crystal polymer
present in the laminated sandwich structure forming the starting material
of the stretching step, i.e., by using thicker or thinner starting films,
and adjusting the amount of stretch to provide the desired increase in
planar surface area, the thickness of the liquid crystal polymer film can
be reduced to 25 micrometers or less, and can be as thin as 2 micrometers.
Furthermore, the liquid crystal polymer film or layer remains intact, does
not have holes or tears, and has a very uniform thickness.
After the stretching step the laminate sandwich structure should also be
heat treated to stabilize dimensional and physical properties, and reduce
strains in the liquid crystal polymer film. The heat treatment is done
with the material restrained to prevent shrinking. Heat treatment
temperature will vary according to the thermotropic liquid crystal polymer
used, but is generally in the range from slightly above the glass
transition temperature (Tg) to slightly below the melt range of the liquid
crystal polymer.
The support membranes can also be removed from one or both surfaces of the
stretched laminated sandwich structure to produce a very thin
free-standing liquid crystal polymer film with excellent surface finishes,
as described earlier.
The free-standing liquid crystal polymer films and laminated sandwich
structures made by the process of the invention provide relatively
balanced physical properties, are thin and light in weight and,
furthermore, have strength, flexibility, thermal expansion, and liquid and
gas barrier characteristics that make them highly desirable for use as
printed circuit board and other electronic substrates, and the like.
TEST DESCRIPTIONS
Tensile Test
Tensile strength and tensile elongation were measured in accordance with
Japanese Industrial Standard JIS K 7127.
Measurement values are given in kg/mm.sup.2, and are shown in Table 1.
Surface Roughness
Surface roughness was measured by a profilometer, Model SURFCOM 570A or
Model SURFCOM 1500A, made by Tokyo Seimitsu Co.
Roughness units, Ra, are given in micrometers and, in accordance with
Japanese Industry Standard JIS B0601, represent the arithmetic mean of the
absolute value of the deviation from the center line.
Surface Layer Separation
The sample surface is softly rubbed with sandpaper and examined visually to
determine if fibers have been created or raised from the surface. The
results are reported simply as "yes" (fibers present) or "no" (no fibers
present).
EXAMPLE 1
As shown in FIG. 1, two support membranes of porous expanded
polytetrafluoroethylene were strung over the surface and through the nip
of a pair of horizontally-mounted heated metal rolls. The porous expanded
polytetrafluoroethylene membranes had a nominal pore size of 0.2
micrometers, a pore volume of 80%, and was 40 micrometers thick.
The heated calender rolls had a working surface of 20 mm diameter and 600
mm length. The nip opening between the rolls was set at 250 micrometers
and the surface temperature of the rolls was 320.degree. C.
A thermotropic liquid crystal polymer (Vectra.RTM. A-950, supplied by
Polyplastics Co.) was melt-extruded directly into a melt region between
the support membranes to form a puddle of melted liquid crystal polymer
material above the nip opening. A conventional uniaxial screw extruder
(screw diameter: 50 mm) and T-die was used. The T-die had a lip length of
500 mm; lip clearance of 1 mm; and was operated at a temperature of
300.degree. C.
The support membrane and liquid crystal polymer material were laminated to
form a sandwich structure by passage through the nip between the rolls at
a speed of about 2 meters/minute, and at an applied pressure of about 3
kg/cm. The laminated sandwich structure was then passed over 50 mm
diameter cooling rolls, operated at a temperature of 150.degree. C., to
solidify the liquid crystal polymer film formed between the support
membranes, and taken up on a roll. A portion of the roll of the laminated
sandwich structure was heat treated at a temperature of 240.degree. C. for
10 minutes, after which the porous polytetrafluoroethylene support
membranes were peeled off, and the free-standing liquid crystal polymer
film was spooled up.
A liquid crystal polymer film having a thickness of 230 micrometers was
produced.
The liquid crystal polymer film was examined and found to not have tears or
holes, and there was no evidence of surface layer separation. Samples were
prepared for tensile tests. Test results are shown in Table 1.
EXAMPLE 2
As described in Example 1, two support membranes of porous expanded
polytetrafluoroethylene were strung over the surface and through the nip
of a pair of horizontally-mounted heated metal rolls. The porous expanded
polytetrafluoroethylene membranes had a nominal pore size of 0.2
micrometers, a pore volume of 80%, and was 40 micrometers thick.
The heated calender rolls had a working surface of 20 mm diameter and 600
mm length. The nip opening between the rolls was set at 250 micrometers
and the surface temperature of the rolls was 320.degree. C.
A sheet of thermotropic liquid crystal polymer (Vectra .RTM. A-950,
supplied by Polyplastics Co.) was melt-extruded, cooled and solidified on
cooling rolls, and taken up. A conventional uniaxial screw extruder (screw
diameter: 50 mm) and T-die was used. The T-die had a lip length of 500 mm;
lip clearance: of 1 mm; and was operated at a temperature of 300.degree.
C.
The sheet of liquid crystal polymer thus produced had a poor balance of
physical properties in the machine direction and transverse direction, and
it had tears at several locations in the machine direction. The sheet of
solid liquid crystal polymer, in lieu of the molten extruded liquid
crystal polymer described in Example 1, was fed into the melt region
between the heated metal rolls, melted to form a puddle above the nip
opening, and further processed as described in Example 1.
A liquid crystal polymer film having a thickness of 230 micrometers was
produced.
The liquid crystal polymer film was examined and found to not have tears or
holes, and there was no evidence of surface layer separation. Samples were
prepared for tensile tests. Test results are shown in Table 1.
EXAMPLE 3
A portion which was not heat treated of the roll of the laminated sandwich
structure described in Example 1 was sequentially stretched biaxially in a
stretching apparatus having a first section of differential speed rolls
for machine direction stretching, and a tenter frame section for
transverse direction stretching. The stretching zone was operated at
295.degree. C. The laminated sandwich structure was stretched an amount of
3:1 in each direction, at a stretch rate of 100% per minute. The stretched
laminated sandwich structure was cooled to solidify the liquid crystal
polymer, and the material heat treated at 240.degree. C. for 10 minutes.
The support membranes were then peeled off, and the free-standing liquid
crystal polymer film spooled up.
A liquid crystal polymer film having a thickness of 25 micrometers was
produced.
The liquid crystal polymer film was examined and found to not have tears or
holes, and there was no evidence of surface layer separation. Samples were
prepared for tensile tests. Test results are shown in Table 1.
EXAMPLE 4
A laminated sandwich structure was made as described in Example 3, except
that the amount of stretch was 9:1 in each direction, and the support
membranes were not removed after heat treatment.
A laminated sandwich structure having a liquid crystal polymer film layer 3
micrometers thick was produced.
The liquid crystal polymer film was examined and found to not have tears or
holes, and there was no evidence of surface layer separation. Samples were
prepared for tensile tests. Test results are shown in Table 1.
Comparative Example 1
A thermotropic liquid crystal polymer (Vectra.RTM. A-950 resin, supplied by
Polyplastics Co.) was extruded and blown at a ratio of 1.2:1 to form a
film. A 50 mm diameter uniaxial screw extruder with a rotating die (Type
304 SS) having a 100 mm orifice was used. Extrusion conditions were: die
rotation rate-7 RPM; die temperature-300.degree. C., The film was cooled
by forced air, heat treated at 240.degree. C. for 10 minutes, and taken
up.
A liquid crystal polymer film having a thickness of 25 micrometers was
produced.
The liquid crystal polymer film was examined and found to not have tears or
holes, however, it was noted that surface layer separation had taken
place. Samples were prepared for tensile tests. Test results are shown in
Table 1.
Comparative Example 2
A liquid crystal polymer film was prepared as described in Comparative
Example 1, except that the blow ratio was 3.6 to 1.
Comparative Example 2 was torn in several places after stretching, and was
not further tested.
EXAMPLE 5
To illustrate surface finish characteristics obtainable with the process of
the invention four liquid crystal polymer films were prepared as described
in Example 1, except that for one film nonporous polyimide membranes were
used as support membranes, and for the remaining three remaining films
porous expanded polytetrafluoroethylene films having nominal pore sizes of
1.0, 0.5, and 0.2 micrometers were used as support membranes. Samples of
each of the liquid crystal polymer films were taken for surface roughness
measurements. The surface roughness measurements are shown in Table 2.
Portions of the above four liquid crystal polymer films were then stretched
and further processed as described in Example 3. Additional samples of the
stretched films were taken for surface roughness measurements. The surface
roughness measurements are shown in Table 2.
For comparative purposes, surface roughness measurements of the film of
Comparative Example 1 are included in Table 2.
TABLE 1
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Tensile
Surface Strength
Thickness Layer kg/mm.sup.2
Example .mu.m Tears Separation
MD TD
______________________________________
1 230 none no 22 20
2 230 none no 21 22
3 25 none no 28 27
4 3 none no 32 32
Comp. Ex. 1
25 none yes 23 23
Comp. Ex. 2
-- several -- -- --
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TABLE 2
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Surface Roughness
Support Film Ra (.mu.m)
______________________________________
unstretched
polyimide 0.06
porous PTFE (1.0 .mu.m)
0.37
porous PTFE (0.5 .mu.m)
0.23
porous PTFE (0.2 .mu.m)
0.15
stretched
polyimide
0.03
porous PTFE (1.0 .mu.m)
0.15
porous PTFE (0.5 .mu.m)
0.09
porous PTFE (0.2 .mu.m)
0.07
Comparative Example 1 2.4
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